Refractoriness to melatonin and short daylengths in early seasonal quiescence in the Bennet's wallaby (Macropus rufogriseus rufogriseus)

Abstract

Adult female Bennet's wallabies were treated with reductions in daylength, melatonin implants or injections of melatonin 2 h before dusk in early or mid-seasonal reproductive quiescence. In early reproductive quiescence (5 weeks after the winter solstice) reproductive quiescence did not end in response to 3 or 5 h of reduced daylength or in response to injections (400 ng/kg) or implants (0.5 g in a Silastic rubber envelope) of melatonin. Reductions in daylength at this time of year did, however, result in an extension of the circadian pattern of melatonin secretion. In mid-reproductive quiescence (21 weeks after the winter solstice) treatment with a 5 h reduction in daylength, melatonin injections administered 2 h before dusk or melatonin implants did result in the termination of reproductive quiescence and reactivation of the quiescent corpus luteum within a period of 5 days. The results of these experiments indicate that, in early reproductive quiescence, the Bennet's wallaby is refractory to the influence of reduced daylength or melatonin, although capable of responding to such reduced days in terms of an increased duration of melatonin secretion. Bennet's wallabies therefore exhibit a refractoriness to short days similar to that of some seasonal eutherians although it remains to be established whether this refractory response is the cause of the transition to seasonal reproductive quiescence.